The tailings which result from flotation or other wet techniques for separating minerals from their ores are in the form of a slurry of the tailings in water. To dispose of these tailings, the slurry is commonly pumped to a tailings dam. Tailings dams are reservoirs where the tailings settle out from the water onto the bed of the reservoir and the water is conducted away from the reservoir and is either recirculated to the separation plant or is disposed of.
To ensure that the water leaving the dam contains a minimum of tailings, the flow of water from a dam takes place from its surface through a decant which comprises a pipe which has an open upper end and extends downwards through the slurry and through any tailings already deposited in the dam to a further pipe which extends laterally under a wall or other barrier which holds the slurry in the dam.
Decants commonly consist of a straight vertical pipe which is gradually extended upwards over a long period of time as the dam fills with deposited tailings and accordingly the water level in the dam gradually rises.
It has been found that such decants quite frequently fail in compression over a period of years and, if this happens, it is necessary to plug the decant and to install a new decant through the slurry and downwards through the deposited tailings and usually to install a new laterally extending pipe as well. The expense of doing this is very great.
The failure of the decants in compression occurs because the tailings build up around and are pressed against the outside surface of the decant and the tailings are of such a nature that they exert a very substantial frictional grip on the decant. As the tailings at the bottom of the dam are consolidated by the deposition of more tailings on top of them, a downward frictional force is applied to the decant and this, in the course of time, becomes so large that the decant is stressed axially to such an extent that it is crushed. The pipe of the decant commonly has a supporting structure such as a surrounding reinforced concrete casing, but even so failure still occurs.
Attempts have been made to overcome this problem by providing the decant with flanged joints fitted with axially compressible rubber sealing rings. As the tailings move downwards as they consolidate, the pipe is also able to move downwards by compression of the sealing rings. However the downward movement of the pipe is progressive, being a minimum at the top of the deposited tailings and a maximum at the bottom of the decant where the decant joins the laterally extending pipe. The movement towards the bottom of the decant tends to be so great that the rubber sealing rings are overstressed and are broken. The deposited tailings are permeable to water and accordingly water leaks into the decant through the joint where the sealing ring is damaged and carries tailings into the decant with it. This means that the water discharged from the decant is severely polluted and, because it is impossible to replace the sealing ring which is far below the surface of the deposited tailings, it is again necessary to plug the decant and construct a new one.
The aim of the present invention is to overcome the problem of destruction of the decants of tailings dams and to this end, according to this invention, the decant of a tailings dam comprises a pipe which is formed as a coil with axially spaced turns one above the other, the end of the bottom turn being connected to a laterally extending water outlet pipe or other duct and the end of the top turn being open for the flow into it of water from the dam, and the turns being of such a size in relation to the cross-sectional area and wall thickness of the pipe from which they are formed that the coil is compressed axially, as, in use of the dam, tailings are consolidated around the decant, without overstressing the pipe.
Thus the decant in effect forms a coiled compression spring which is compressed by the consolidation of the tailings as the tailings build up on the bed of the dam around the decant and the compression takes place within the elastic limits of the pipe from which the spring is formed. It is possible in this way to absorb very large downward movements of the tailings as they consolidate without causing any damage to the decant.
The turns of the coil may be of any shape which is consistent with their being able to move towards each other without overstressing the pipe and the pipe may also be of any cross-sectional shape with the same proviso. Preferably, however, the pipe is of circular cross-section and the turns of the coil to which the pipe is formed are also preferably circular.
The coil may be a helix except for its bottom turn which preferably lies in a plane perpendicular to the axis of the coil so that it can rest on a horizontal bed to support the turns of the coil above it.
However, as the tailings gradually build up from the bottom of the dam around the decant, and additional lengths of pipe forming additional turns are added to extend the decant upwards, the downward movement of the lower turns of the coil will be greater than the downward movement of the turns nearer the top of the coil. If the coil is helical, the turns must be of sufficiently large diameter to absorb a maximum movement at the bottom of the coil and this means that the higher coils are of a larger diameter than is necessary to absorb the movement to which they are subjected. Accordingly the decant pipe as a whole is longer than is necessary and this increases the cost and also increases the frictional head loss of the water flowing through it. Preferably therefore the pipe is formed into an upwardly tapering conical spiral so that each successive turn of the coil is of a smaller diameter than the turn immediately below it. In this way it is possible to make each turn of the coil of a diameter which is just sufficient to enable the pipe in the turn to withstand the downward deflection of the turn relative to the turn below it.
The axial spacing between adjacent turns of the coil may be small or larger in dependence upon the particular stress conditions which are anticipated. These will depend again upon the diameter or thickness of the pipe and upon the nature of the tailings which will affect the amount by which they are consolidated as the tailings build up.
The major strain in a decant pipe formed into a coil in accordance with the invention is torsional and in general this is much greater than the strain due to bending or shear.
As the tailings are consolidated around the pipe, the pipe, if initially of circular cross-section, may become slightly oval with its major axis horizontal, but the passive resistance of the tailings surrounding the sides of the pipe would be very great and would tend to resist any such distortion of the cross-section of the pipe.
The wall thickness of the pipe should be made as thin as is consistent with the stresses to which it is subjected, bearing in mind also the consideration that the pipe is subjected to some wear from any tailings remaining in the water discharged from the dam through the decant.
The coil is built up from individual sections or lengths of piping as the level of the tailings dam rises as has already been mentioned and, since tailings dams are generally in situations where skilled labour is scarce or non-existent and the conditions are unsuitable for high quality welding, the joints between adjacent sections of the piping are preferably mechanical.
The use of heavy flanged joints may introduce large secondary stresses in the pipe since the bending of the pipe will be inhibited by the flanges and accordingly if flange joints are used, these should preferably be made as thin as as practicable with thin sealing gaskets or sealing by means of paste sandwiched between the flanges. The use of a large number of small bolts around each flange is preferable to the use of a smaller number of larger bolts.
In cases where the large torsional strains are expected, it is preferable to avoid the use of flanged joints and instead the joints between adjacent sections of piping preferably comprise a locking ring which surrounds and is welded to the end of each pipe section. Each locking ring has a series of circumferentially spaced and axially projecting teeth which mesh with the teeth of the locking ring at the end of the adjacent pipe section. The meshing of the teeth transmits torsional stresses in one section of piping to the adjacent section of piping. The sections of piping are held together by straps which surround the two meshing locking rings and have radially inwardly directed side edges which engage over the edges of the locking ring remote from the teeth and so prevent the locking rings, and hence the pipe sections, from moving axially away from each other. Each of the straps is formed in two semi-circular sections which are fitted around the locking rings and are then bolted to each other to form a continuous circular strap surrounding the locking ring at each joint. The teeth on the locking rings may be of a rectangular wave form, but they are preferably shaped so that the flanks of the teeth taper slightly towards the free ends of the teeth and similarly spaces between adjacent teeth taper towards the roots of the teeth. This tapering arrangement makes it easier to bring the teeth of the two locking rings at a joint into mesh with each other.
The coiled pipe forming the decant may be free-standing on the bed of the tailings dam so at no time the pipe extends very far above the surface of the tailings deposited in the dam. As the length of the coil is increased by the addition of further turns, more of the turns at the bottom of the coil become supported by the deposited tailings. Even if the axis of the coil does deflect slightly from the vertical in a very high dam, which may, for example, be up to 100 m high, the stresses arising from the deflection are not likely to be very significant. Nevertheless under some circumstances the decant may include some additional structure for supporting the coil provided of course that this structure does not inhibit the compression of the coil. Generally speaking all that is necessary is some temporary support of the turn or turns of the coil which project in the slurry above the surface of the consolidated tailings in the dam up to the water level. This temporary structure is dismantled and replaced from time to time as further turns are added to the coil and the level of the consolidated tailings rises.